(1) Field of the Invention
This invention relates to an improved process and improved compositions for imparting flame retardance to cotton and other fibrous cellulosic materials. More specifically, this invention relates to the improved process and compositions resulting from incorporating formaldehyde donors into the ammonium hydroxide curing formulations and applying these to the sensitized fabric by means of a "wet transfer" padding technique, thus providing an improved process for the preparation of cotton fabric and other cellulosic material with fire retardant properties.
(2) Description of the Prior Art
Methylol phosphorus based polymers which have nitrogen atoms incorporated in the polymers are known to have excellent flame-retardant properties. Such polymers are particularly suitable in the treatment of cellusosic materials such as cotton, rayon, jute, ramie, paper, cotton-blended fabrics, etc. Numerous processes have been developed for treating cellulosic materials with such polymers. U.S. Pat. No. 2,668,096 describes a process wherein cotton fabric is impregnated with an aqueous solution of tetrakis(hydroxymethyl)phosphonium chloride (Thpc), and a cyclic-nitrogen containing compound, and then heated at an elevated temperature. Although this particular process produces good fire-retardance it tends to weaken the tensile properties of the fabric and make it unsuitable for many applications where strong fabric are required.
U.S. Pat. No. 3,236,676 discloses a process wherein ammonia reacts with partially or completely neutralized Thpc to produce adducts and polymers. Cellulosic materials are impregnated with partially or fully neutralized Thpc and heated at a high temperature or alternately at a lower temperature for a longer time period to place the Thpc in the cellulosic fiber. Subsequently, the thoroughly dried textile is exposed to ammonia.
U.S. Pat. No. 3,607,356 discloses a process wherein the cellulosic material is impregnated with tetrakis(hydroxymethyl)phosphonium hydroxide (THPOH) or fully neutralized Thpc (pH of 7 or above), then the cellulosic material is partially dried and exposed to ammonia gas to produce insoluble flame-retardant polymer in the textile.
The efficiency of polymer formation in a cellulosic textile is very low when the textile is impregnated with Thpc and the dried textile is subsequently exposed to ammonia. Thpc, which is strongly acidic (pH about 2) reacts slowly with ammonia, and with amides, to produce insoluble polymers. Reaction is accelerated or promoted by partial or complete neutralization (pH about 7.5) with a base. Therefore, such a process without neutralization is not a satisfactory commercial method for imparting flame resistance to textiles. Besides, much of the flame retardant is removed by laundering. When Thpc is partially or fully neutralized with a base and applied to fabric as shown, polymer formation is efficient when the dried or partially dried fabric is exposed to ammonia gas.
The standard procedure used for the commercial production of THPOH--NH.sub.3 finished flame retardant cotton fabrics involves the use of ammonia gas as curing agent. From the viewpoint of the textile industry, the use of ammonia gas poses several disadvantages. First, the industry is currently equipped to treat fabric using aqueous systems to impart finishing chemicals to fabric. In order to use ammonia gas, special curing chambers must be designed, built and placed in a finishing line. To permit operation at modern finishing speeds, textile manufacturers must use large and costly chambers. Because the fabric is passing through these chambers in a continuous manner, it is not possible to completely seal these chambers. As such, there is a tremendous loss of ammonia from the chambers so that the plant atmosphere poses a health hazard to the workers. Indeed, this odor is often so permeating that only people wearing gas masks can stay in the immediate vicinity of the operation. The rapid escalation of ammonia prices has made the loss of ammonia in the air an economic consideration as well as a health factor. Also, a basic gas floating around a textile mill poses an added problem because most operations are acid catalyzed and there is a potential for interference with these catalysts. In addition, the necessity for gas to penetrate to all regions of the fabric would appear to raise a problem with respect to evenness of treatment in high speed operations. Modern flammability test requirements are such that each and every sample of treated fabric must be able to pass the bone dry flammability test, i.e., Children's Sleepwear Standard, DOC FF3-71, Fed. Register, July 29, 1971. Finally, because the use of gas finishing techniques represents an entirely new technology to most textile companies, there is a reluctance on their part to produce products which require new technology and new equipment.
For these reasons, it was deemed advantageous for the ammonia to be added to an aqueous phase in the form of ammonium hydroxide or other ammonium salts. Previous attempts to do this by running a fabric treated with THPOH into an aqueous system containing ammonium hydroxide were unsatisfactory for several reasons. First, the THPOH is water soluble and much of the THPOH on the fabric would be leached into the aqueous bath, where a polymer quickly forms. This polymer is deposited both in the bath and on the squeeze rolls, thus interfering with bath life and roller efficiency. A second consequence of this leaching is that the treated fabric now has insufficient polymer to pass the flammability tests. To demonstrate this several experiments were performed which would indicate typical results from the various approaches. Thus, two samples of cotton fabric were padded with 35% THPOH and dried. One sample was ammoniated as described by Beninate et al. U.S. Pat. No. (3,607,356) and gave a phosphorus analysis of 5.8% (subsequent to oxidation). By contrast the companion sample was padded with ammonium hydroxide and then oxidized. This latter sample had a phosphorus analysts of 1.5%. A phosphorus content of 3.5-4.0% is required to produce a fabric which will pass the modern flammability test initially and after 50 launderings. Thus, it can be seen that taking a sample of THPOH fabric and padding or putting it through a bath of ammonium hydroxide does not produce a flame-retardant fabric.